Maybe CSI has accurately reported the numbers provided by the exchange, i.e., if somebody made an error, it wasn't CSI.

One way to investigate this possibility is to locate somebody who has historical price data on this contract, from a completely independent source. A data source which is not CSI, which is not somebody that supplies data to CSI, and not somebody that receives data from CSI. Such as perhaps CQG or Bloomberg or ThomsonReuters or TickData.

Then find a way to motivate this person, so that she will provide the data to you.

When you've got data from an independent source, you can check to see whether it also contains the discrepancy you report here.

You may also want to investigate writing your own continuous contract builder software (or hiring someone to do it for you). Then you can design and implement safeguards, sanity-checks, and reality constraints in your rollover algorithm. Some nifty software design ideas are discussed in (this) thread.

Yes, Exchange could have reported bad data.
All exchanges make erroneous reports. Data providers know that and have front end data edit programs to identify potential problems. CSI has a data validation department where the staff does catch and correct a lot of errors.

In this case, the data is in error. Where the error came from? Who knows...

The Open Interest as reported by CSI should have failed simple arithmetic validation.